The present invention relates to a process for the separation of zirconium values from hafnium values in a mixture containing the same.
The uses of zirconium and high zirconium alloys, such as zircaloy-2, zircaloy-4 and zirconium-2.5 wt.% niobium, in nuclear reactor components are well known. Zirconium and high zirconium alloys being especially useful because of zirconium's low absorption of neutrons. Zirconium is found in ores which also contain hafnium values. The hafnium, which has a moderate absorption capacity for neutrons must be essentially completely removed from mixtures with zirconium, if the zirconium, and its alloys are to be useful in nuclear reactor systems. Nuclear specifications for zirconium and high zirconium alloys generally require that the hafnium impurity content be kept at or below 100 ppm. The hafnium, in its commercially pure state is also useful in specific portions of a nuclear reactor system, where absorption of neutrons is desired. Commercial purity hafnium generally contains about 4 wt.% zirconium. Thus, zirconium and high zirconium alloys are useful as cladding materials, while hafnium is useful in control elements.
The separation of hafnium values from zirconium is one of the more difficult steps in purifying zirconium. These elements are very similar in their physical and chemical properties such as ion size, solubility, reaction chemistry, and the like. Existing commercial processes use solvent extraction of thiocyanate complexes of these metals to produce zirconium having the aforementioned low hafnium impurity level. Problems exist however because of known competing reactions that lead to decomposition of the thiocyanate. The proceeding commercial process is summarized in J. H. Schemel, "ASTM Manual on Zirconium and Hafnium," ASTM Special Technical Publication 639, (1977), of which pages 56 to 59 are hereby incorporated by reference. Other processes for separation of zirconium from hafnium values are discussed in U.S. Pat. No. 3,127,236, which treats a mixture of insoluble compounds such as phosphates or hydroxides with oxalic acid to form oxalato complexes and fractionally precipitates the zirconium and hafnium values from the solution of the complexes, and in U.S. Pat. No. 3,069,232 which uses a saturated solution of ammonium sulfate instead of sulfuric acid to extract hafnium values from the organic phase of a preferrential solvent extraction process for hafnium using a thiocyanate complex.
In a recent patent, U.S. Pat. No. 4,389,292, which is a co-invention of one of the co-inventors herein, and the contents of which are incorporated herein by reference, the .sup.91 ZR isotopic content of zirconium is altered by raising a zirconium chelate ligand, such as a tetraoxalatozirconate, from a ground state to an activated state in the presence of a scavenger that reacts with the activated ligand, and separating the reacted ligand. Activation of the zirconium compound, i.e. the breaking of a bond in the compound which can either recombine or react with the scavenger, is effected either with the use of heat or the use of light.
We have now discovered that hafnium values can be separated from zirconium values, in a mixture containing both, by irradiating a solution of a mixture of organic complexes of these two elements with light having a wavelength that will excite one of the complexes while the other of the complexes will remain stable, with the excited complex then being separated from the solution while the other complex remains in solution.